The present invention is generally related to radio frequency (RF) signal couplers, and more particularly to an offset transmission line coupler for RF signal amplifiers of cellular telephones.
In conventional cellular telephones, RF signal couplers have been implemented by several methods, including by means of capacitive couplers or transmission line couplers. Prior transmission line couplers typically have been implemented by a first transmission line on the top surface of a top layer of a multi-layer circuit board and a second transmission line on the bottom surface of the top layer, directly under and electromagnetically coupled to the first transmission line. A ground plane is disposed on the bottom surface of a bottom layer of the circuit board, which is laminated to the top layer. The top layer of the circuit board is of sufficient thickness to achieve a desired amount of coupling between the first and second transmission lines. However, as the thickness of circuit boards has been decreased to decrease size and weight of cellular telephones, it has been necessary to make the second transmission line thinner and to offset the second transmission line to provide the desired amount of coupling to the first transmission line. However, its is difficult to maintain the desired amount of coupling between the first and second transmission lines, because of the variation in the registration between the transmission line plating on the top and bottom surfaces of the top layer of the circuit board. For example, a variation of plus or minus 0.003 inches in the registration of the circuitry plating will produce a coupling variation of up to plus or minus 3 dB. For the foregoing reasons, there is a need for an improved offset transmission line coupler that provides and maintains a desired amount of coupling over small variations in registration between the transmission line plating.